The semiconductor integrated circuit (IC) industry has experienced exponential growth. Technological advances in IC materials and design have produced generations of ICs where each generation has smaller and more complex circuits than the previous generation. In the course of IC evolution, functional density (i.e., the number of interconnected devices per chip area) has generally increased while geometry size (i.e., the smallest component (or line) that can be created using a fabrication process) has decreased. This scaling down process generally provides benefits by increasing production efficiency and lowering associated costs. Such scaling down has also increased the complexity of processing and manufacturing ICs.
Multi-gate devices have been introduced in an effort to improve gate control by increasing gate-channel coupling, reduce OFF-state current, and reduce short-channel effects (SCEs). Examples of the multi-gate devices include double-gate FET, triple-gate FET, FinFET, omega-gate FET, and gate-all-around (or surround-gate) FET. The multi-gate FETs are expected to scale the semiconductor process technology beyond the limitations of the conventional bulk metal-oxide-semiconductor FET (MOSFET) technology. However, as the transistor structure scales down and becomes three dimensional, the quality of the transistor source and drain epitaxial structure exhibits increased impact on the device performance. Although existing approaches in source and train epitaxial structure formation have been generally adequate for their intended purposes, they have not been entirely satisfactory in all respects.